Hydraulic fracturing operations are often carried out on oil and gas wells to increase the flow of oil and natural gas from the wells. The fracturing fluid creates fractures in the formation and transports and deposits proppants into the fractures. The proppants hold the fractures open after the fracturing fluid flows back into the well. To adequately propagate fractures in subterranean formations, the fracturing fluid should exhibit minimal fluid loss into the formation and should have sufficient viscosity to carry large volumes of proppant into the cracks in the formation formed during fracturing. The fracturing fluid, however, should also readily flow back into the well after the fracturing operation is complete, without leaving residues that impair permeability and conductivity of the formation.
In order to increase the viscosity of fracturing fluids, hydratable high molecular weight polymers such as Guar, polysaccharides, polyacrylamides and polyacrylamide copolymers are sometimes used. The viscosity of the fracturing fluid can be further increased by adding crosslinking compounds to the fluids. The term “crosslink” is used herein to refer to “an attachment of two chains of polymer molecules by bridges, composed of either an element, a group, or a compound that joins certain atoms of the chains by association.” Conventional crosslinking agents such as polyvalent metal ions or borate ions form chemical bonds between the viscosifier polymer molecules which raise the viscosity of the solution. In order to allow the crosslinked fluid to flow back out of the formation and into the well, a breaker is sometimes added to the fracturing fluid to degrade the molecular weight and thereby reduce the viscosity of the fracturing fluid.
Viscoelastic surfactants have also been added to fracturing fluids to increase their viscosity. For example, gels can be formed by the association of hydrophobic portions of surfactants to form micelles or larger associative structures. The micelles or other associative structures increase the viscosity of the base fluid. A “micelle” is as “a colloidal particle composed of aggregates of surfactant molecules.”
During the fracturing operation, the polymers and other compounds used to increase the viscosity of the fracturing fluid form a film over the fracture matrix, referred to as a “filtercake.” The filtercake prevents excessive fluid leakage into or out of the formation. After the fracturing operation is complete, however, as much of the filtercake as possible is removed. Otherwise, the filtercake will impede the flow of oil and gas into the well bore. In particular, filtercakes deposited from conventional fracturing fluids can be difficult to remove and significantly interfere with oil and gas production.
Guar in particular is a commonly used polymer in fracking fluids. Upon degradation, however, Guar tends to form water insoluble particles that create residues that can significantly impede recovery of hydrocarbons from the well. The water insoluble particles are believed to remain in the formation and cause plugging that impairs the permeability of sand or gravel packs.
It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings.